求一条关于时间的sql

Question

求两个时间戳格式的时间所间隔的日数

Answer 1

select date'2012-05-10'-date'2012-01-01'  from dual;

Answer 2

select sysdate - to_date('2012-05-01 12:12:12','yyyy-mm-dd hh24:mi:ss') from dual

Answer 3

select systimestamp - to_timestamp('2012-05-01 12:00:00','yyyy-mm-dd hh24:mi:ss') from dual

Answer 4

select date'2012-05-10'-date'2012-01-01'  from dual